It is not advisable to follow someone you think is a criminal, but it is in no way illegal. If Trayvon hit him first and started banging his head on the pavement then he has every right to stop a potentially deadly attack. It IS a crime to assault someone. What should he do? Just give up and say "Oh well, kids will be kids"?
As far as "stand your ground laws", here is what you give up without such a law: Let's say some Anders Brevik kind of maniac starts shooting you and your family across the street from me. I would likely have my CC weapon on me and be in a position to help. However, without a stand your ground law, I would run away. Sorry, but I can't risk going to jail just to save your kids. Perhaps you could call the cops? Because when seconds count, the cops are just minutes away.
Um that is not correct. If a crime is in progress, you have the right under common law to plead self-defence for any reasonable injury you inflict preventing it. For example, giving a mugger a bloody nose even though he was robbing an old lady, not you, is perfectly legal.
"Stand your ground" takes this a step further. If you think the guy is going to mug the old lady, maybe you can kill him. The "reasonable" test is removed.
In this case, if Trayvon attacked Zimmerman, Zimmerman was well within his rights to kill him both under common law and "stand your ground" reinforces this. On the other hand, if the voice screaming for mercy in the 911 call is that of Trayvon, Zimmerman is in trouble.
Maybe it was Trayvon screaming for assistance while on top of Zman and banging his head on the sidewalk.
I've seen people do that before.